I'm on Windows 10

Noticed the bug in version 0.5.18 and still see it in 0.5.19

Steps to reproduce, for any girl :

1. Set the girl portrait

2. Set the girl body

As soon as the body is set, the portrait is reseted. If you reverse those 2 steps, the body is reseted when the portrait is set. Also, if the body is lost in this way, there is no image in the inventory menu (not even the race template)

There might be a link with the picture set in the inventory window since I noticed this only when this feature appeared.

There's an icon in the lower right that says 'Assign Both'. Uncheck that one and you'll be able to assign only the portrait or body image. I assume it's there for people who a portrait and body images with the same name where it'll automatically assign both. Not sure why it's on by default since as you say, it covers over the other image.